The Early Development of the Snail Biomphalaria Glabrata (Say) and the Origin of the Head Organs

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The cell-lineage of Biomphalaria glabrata has been studied for all cell-lines up to the 130-cell stage. For the head region the origin of the various organs has been traced back to original stem cells. Biomphalaria as a sinistral snail shows the reversed type of cleavage, in which third cleavage is laeotropic. After the formation of the primary mesoblast 4d or M at the 25-cell stage the embryo becomes bilaterally symmetrical. A typical molluscan cross becomes visible at the animal side of the embryo, when this attains a stage of about 45 cells. This cross consists of cells descending from the first quartette of micromeres together with four cells of the second quartette, which form the tips of the arms of the cross. Originally the cross is radially symmetrical, but at the division of the tipcell of the ventral arm, 2b11, the bilateral symmetry becomes definitely established. Apart from the prototroch or velum, which is partly derived from the cells 2b12 and 2b21, the head region originates from cells which form the molluscan cross. Some of these cells stop dividing and form larval structures such as apical plate and head vesicle, whereas other cells continue dividing and form the cephalic plates, developing into adult structures such as eyes, tentacles and cerebral ganglia. The data are discussed in relation to the bilateral symmetry of the head region.